Plasticizing system and rubber composition for tire containing said system

ABSTRACT

Plasticizing system which can be used in particular for the plasticizing of a tire diene elastomer composition containing, in combination:
         a plasticizing hydrocarbon resin, the glass transition temperature of which is greater than 0° C.; and   a tri- or pyromellitate ester corresponding to the formula (I):       

                         
in which the R radicals, which are identical or different, represent a hydrocarbon radical and the R′ radical represents hydrogen or COOR group. Also, a rubber composition incorporating the plasticizing system and the use of such a composition for the manufacture of a tire or tire semi-finished product made of rubber, in particular of a tire tread, exhibiting an improved wear resistance without having a detrimental affect on its wet grip.

The present invention relates to rubber compositions intended in particular for the manufacture of tyre treads; it relates more particularly to plasticizing systems which can be used for the plasticizing of such compositions.

As is known, a tyre tread has to meet a large number of often conflicting technical requirements, including a low rolling resistance, a high wear resistance and a high grip on both the dry road and the wet road.

This compromise in properties, in particular from the viewpoint of the rolling resistance and the wear resistance, was able to be improved in recent years with regard to energy-saving “Green Tyres”, intended in particular for passenger vehicles, by virtue of the use of novel weakly hysteretic rubber compositions having the characteristic of being reinforced predominantly by specific inorganic fillers described as reinforcing, in particular by highly dispersible silicas (HDS), capable of rivaling, from the viewpoint of the reinforcing power, conventional tyre-grade carbon blacks. Thus, today, these reinforcing inorganic fillers are gradually replacing carbon blacks in tyre treads, all the more so as they have another known virtue, that of increasing the grip of tyres on wet, snowy or icy roads.

The improvement in the grip/wear resistance compromise remains, however, a constant preoccupation of designers of tyres, whether the latter comprise treads comprising silica or carbon black as filler.

The use is known, to promote the properties of wear and abrasion resistance and/or of grip of tyre treads, of plasticizing hydrocarbon resins in their rubber compositions, as described, for example, in Patents or Patent Applications U.S. Pat. No. 3,927,144, GB-A-2 178 046, JP-A-61-190538, JP-A-09-328577, WO-A-91/18947, WO-A-02/088238, WO-A-02/072688 and WO-A-02/072689.

Improvements to the above solutions have been obtained by combining these plasticizing hydrocarbon resins with certain specific plasticizing agents.

In particular, in order to further increase the wear and abrasion resistance of tyre treads, plasticizing systems have been provided which comprise, in combination, non-aromatic oils of the MES or TDAE type with terpene hydrocarbon resins, such as polylimonene, or with C₅ fraction/vinylaromatic copolymer hydrocarbon resins (see Applications WO-A-2005/087859 and WO-A-2006/061064).

In point of fact, on continuing their research, the Applicant Companies have discovered a novel plasticizing system based on a plasticizing hydrocarbon resin which makes it possible, in comparison with a combination of a hydrocarbon resin and of an IVIES oil, to further improve the wear resistance of tyres without having a detrimental affect on their wet grip.

Thus, a first subject-matter of the invention is a rubber composition comprising at least one diene elastomer, one reinforcing filler and one plasticizing system, characterized in that the said plasticizing system comprises, in combination:

-   -   a plasticizing hydrocarbon resin, the glass transition         temperature (Tg) of which is greater than 0° C.; and     -   a tri- or pyromellitate ester corresponding to the formula (I):

-   -   in which the R radicals, which are identical or different,         represent a hydrocarbon radical and the R′ radical represents         hydrogen or the COOR group.

The invention also relates, per se, to a plasticizing system which can be used for the plasticizing of a diene rubber composition, the said system comprising, in combination, a plasticizing hydrocarbon resin, the Tg of which is greater than 0° C., and a compound of formula (I), and also to the use of such a system for the plasticizing of a tyre diene rubber composition.

Another subject-matter of the invention is a process for preparing a rubber composition exhibiting an improved wear resistance, this composition being based on a diene elastomer, on a reinforcing filler and on a plasticizing system, the said process comprising the following stages:

-   -   incorporating in a diene elastomer, in a mixer:         -   a reinforcing filler;         -   a plasticizing system;     -   everything being kneaded thermomechanically, in one or more         goes, until a maximum temperature of between 110° C. and 190° C.         is reached;     -   cooling the combined mixture to a temperature of less than 100°         C.;     -   subsequently incorporating:         -   a crosslinking system;     -   kneading everything up to a maximum temperature of less than         110° C.;     -   extruding or calendering the rubber composition thus obtained;         and being characterized in that the plasticizing system         comprises at least:     -   one plasticizing hydrocarbon resin, the glass transition         temperature (Tg) of which is greater than 0° C.; and     -   a tri- or pyromellitate ester corresponding to the above formula         (I).

Another subject-matter of the invention is the use of a composition according to the invention for the manufacture of a finished article or of a semi-finished product made of rubber intended for any ground-contact motor vehicle system, such as tyre, internal safety support for a tyre, wheel, rubber spring, elastomeric joint, other suspension element and vibration damper.

A particular subject-matter of the invention is the use of a composition according to the invention for the manufacture of tyres or semi-finished products made of rubber intended for these tyres, these semi-finished products preferably being chosen from the group consisting of treads, crown reinforcing plies, sidewalls, carcass reinforcing plies, beads, protectors, underlayers, rubber blocks and other internal rubbers, in particular decoupling rubbers, intended to provide the bonding or the interface between the abovementioned regions of the tyres.

A more particular subject-matter of the invention is the use of a composition according to the invention for the manufacture of a tyre tread.

Another subject-matter of the invention is the tyres themselves and the semi-finished products, in particular tyre treads, when they comprise a rubber composition in accordance with the invention.

The tyres of the invention are particularly intended to equip motor vehicles, such as passenger vehicles, SUVs (Sport Utility Vehicles), two-wheel vehicles (in particular motor cycles), aeroplanes, such as industrial vehicles chosen from vans, heavy-duty vehicles—that is to say, underground, bus, heavy road transport vehicles (lorries, tractors, trailers) or off-road vehicles, such as heavy agricultural vehicles or earthmoving equipment—, and other transportation or handling vehicles.

The invention and its advantages will be easily understood in the light of the description and exemplary embodiments which follow.

I.—MEASUREMENTS AND TESTS

The rubber compositions are characterized, before and after curing, as indicated below.

I-1. Mooney Plasticity

Use is made of an oscillating consistometer as described in French Standard NF T 43-005 (1991). The Mooney plasticity measurement is carried out according to the following principle: the composition in the raw state (i.e., before curing) is moulded in a cylindrical chamber heated to 100° C. After preheating for one minute, the rotor rotates within the test specimen at 2 revolutions/minute and the working torque for maintaining this movement is measured after rotating for 4 minutes. The Mooney plasticity (ML 1+4) is expressed in “Mooney unit” (MU, with 1 MU=0.83 newton·meter).

I-2. Rheometry

The measurements are carried out at 150° C. with an oscillating disc rheometer, according to standard DIN 53529—part 3 (June 1983). The change in the rheometric torque as a function of time describes the change in the stiffening of the composition as a result of the vulcanization reaction. The measurements are processed according to standard DIN 53529—part 2 (March 1983): t_(i) is the induction time, that is to say the time necessary for the start of the vulcanization reaction; t_(α) (for example t₉₀) is the time necessary to achieve a conversion of α %, that is to say α % (for example 90%) of the difference between the minimum and maximum torques. The conversion rate constant, denoted K (expressed as min⁻¹), which is first order, calculated between 30% and 80% conversion, which makes it possible to assess the vulcanization kinetics, is also measured.

I-3. Shore A Hardness

The Shore A hardness of the compositions after curing is assessed in accordance with Standard ASTM D 2240-86.

I-4. Tensile Tests

These tests make it possible to determine the elasticity stresses and the properties at break. Unless otherwise indicated, they are carried out in accordance with French Standard NF T 46-002 of September 1988. The “nominal” secant moduli (or apparent stresses, in MPa) or the “true” secant moduli (reduced in this case to the real cross section of the test specimen) are measured in second elongation (i.e., after a cycle of accommodation) at 10% elongation (denoted “M10” and “E10” respectively), 100% elongation (“M100” and “E100” respectively) and 300% elongation (“M300” and “E300” respectively). All these tensile measurements are carried out under the standard conditions of temperature (23±2° C.) and hygrometry (50±5% relative humidity) according to French Standard NF T 40-101 (December 1979). The breaking stresses (in MPa) and the elongations at break (in %) are also measured, at a temperature of 23° C.

I-5. Dynamic Properties

The dynamic properties are measured on a viscosity analyser (Metravib VA4000) according to Standard ASTM D 5992-96. The response of a sample of vulcanized composition (cylindrical test specimen with a thickness of 4 mm and with a cross section of 400 mm²), subjected to a simple alternating sinusoidal shear stress, at a frequency of 10 Hz, at a temperature of 40° C., is recorded. A strain amplitude sweep is carried out from 0.1% to 50% (outward cycle) and then from 50% to 1% (return cycle); the maximum value of the loss factor, denoted tan(δ)_(max), is recorded for the return cycle.

I-6. Tests on Tyres

A) Rolling Resistance

The rolling resistance is measured on a test drum according to the ISO 87-67 (1992) method. A value greater than that of the control, arbitrarily set at 100, indicates an improved result, that is to say a lower rolling resistance.

B) Wear Resistance

The tyres are subjected to actual on-road running on a specific motor vehicle until the wear due to the running reaches the wear indicators positioned in the grooves of the tread. A value greater than that of the control, arbitrarily set at 100, indicates an improved result, that is to say a greater mileage traveled.

C) Wet Grip

In order to assess the wet grip performances, the behaviour of the tyres fitted to a specific motor vehicle travelling, under limit speed conditions, on a circuit with a great many bends and which is sprayed so as to keep the ground wet is analysed. The minimum time necessary to travel the entire circuit is measured; a value greater than that of the control, arbitrarily set at 100, indicates an improved result, that is to say a shorter journey time.

II. DETAILED DESCRIPTION OF THE INVENTION

The rubber composition according to the invention, which can be used in particular for the manufacture of a tyre or of a tyre tread, comprises at least one diene elastomer, one reinforcing filler and one specific plasticizing system.

Unless expressly indicated otherwise, the percentages shown in the present patent application are % by weight.

II-1. Diene Elastomer

The term “diene” elastomer or rubber should be understood as meaning, in a known way, an (one or more are understood) elastomer resulting at least in part (i.e., a homopolymer or a copolymer) from diene monomers (monomers carrying two carbon-carbon double bonds which may or may not be conjugated).

These diene elastomers can be classified into two categories: “essentially unsaturated” or “essentially saturated”. The term “essentially unsaturated” is understood to mean generally a diene elastomer resulting at least in part from conjugated diene monomers having a level of units of diene origin (conjugated dienes) which is greater than 15% (mol %); thus it is that diene elastomers such as butyl rubbers or copolymers of dienes and of α-olefins of EPDM type do not come within the preceding definition and can in particular be described as “essentially saturated” diene elastomers (low or very low level of units of diene origin, always less than 15%). In the category of “essentially unsaturated” diene elastomers, the term “highly unsaturated” diene elastomer is understood to mean in particular a diene elastomer having a level of units of diene origin (conjugated dienes) which is greater than 50%.

Given these definitions, the term diene elastomer capable of being used in the compositions in accordance with the invention is understood more particularly to mean:

-   -   (a)—any homopolymer obtained by polymerization of a conjugated         diene monomer having from 4 to 12 carbon atoms;     -   (b)—any copolymer obtained by copolymerization of one or more         conjugated dienes with one another or with one or more         vinylaromatic compounds having from 8 to 20 carbon atoms;     -   (c)—a ternary copolymer obtained by copolymerization of ethylene         and of an α-olefin having 3 to 6 carbon atoms with a         non-conjugated diene monomer having from 6 to 12 carbon atoms,         such as, for example, the elastomers obtained from ethylene and         propylene with a non-conjugated diene monomer of the         abovementioned type, such as, in particular, 1,4-hexadiene,         ethylidenenorbornene or dicyclopentadiene;     -   (d)—a copolymer of isobutene and of isoprene (butyl rubber) and         also the halogenated versions, in particular chlorinated or         brominated versions, of this type of copolymer.

Although it applies to any type of diene elastomer, a person skilled in the art of tyres will understand that the present invention is preferably employed with essentially unsaturated diene elastomers, in particular of the type (a) or (b) above.

The following are suitable in particular as conjugated dienes: 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C₁-C₅ alkyl)-1,3-butadienes, such as, for example, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene or 2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene, 1,3-pentadiene or 2,4-hexadiene. The following, for example, are suitable as vinylaromatic compounds: styrene, ortho-, meta- or para-methylstyrene, the “vinyltoluene” commercial mixture, para-(tert-butyl)styrene, methoxystyrenes, chloro styrenes, vinylmesitylene, divinylbenzene or vinylnaphthalene.

The copolymers can comprise between 99% and 20% by weight of diene units and between 1% and 80% by weight of vinylaromatic units. The elastomers can have any microstructure which depends on the polymerization conditions used, in particular on the presence or absence of a modifying and/or randomizing agent and on the amounts of modifying and/or randomizing agent employed. The elastomers can, for example, be block, random, sequential or microsequential elastomers and can be prepared in dispersion or in solution; they can be coupled and/or star-branched or also functionalized with a coupling and/or star-branching or functionalization agent.

The following are suitable: polybutadienes, in particular those having a content (molar %) of 1,2-units of between 4% and 80% or those having a content (molar %) of cis-1,4-units of greater than 80%, polyisoprenes, butadiene/styrene copolymers and in particular those having a styrene content of between 5% and 50% by weight and more particularly between 20% and 40%, a content (molar %) of 1,2-bonds of the butadiene part of between 4% and 65% and a content (molar %) of trans-1,4-bonds of between 20% and 80%, butadiene/isoprene copolymers, in particular those having an isoprene content of between 5% and 90% by weight and a glass transition temperature (Tg, measured according to ASTM D3418) of −40° C. to −80° C., or isoprene/styrene copolymers, in particular those having a styrene content of between 5% and 50% by weight and a Tg of between −25° C. and −50° C. In the case of butadiene/styrene/isoprene copolymers, those having a styrene content of between 5% and 50% by weight and more particularly of between 10% and 40%, an isoprene content of between 15% and 60% by weight and more particularly between 20% and 50%, a butadiene content of between 5% and 50% by weight and more particularly of between 20% and 40%, a content (molar %) of 1,2-units of the butadiene part of between 4% and 85%, a content (molar %) of trans-1,4-units of the butadiene part of between 6% and 80%, a content (molar %) of 1,2-plus 3,4-units of the isoprene part of between 5% and 70% and a content (molar %) of trans-1,4-units of the isoprene part of between 10% and 50%, and more generally any butadiene/styrene/isoprene copolymer having a Tg of between −20° C. and −70° C., are suitable in particular.

To sum up, the diene elastomer of the composition according to the invention is preferably chosen from the group of the highly unsaturated diene elastomers consisting of polybutadienes (abbreviated to “BR”), synthetic polyisoprenes (IR), natural rubber (NR), butadiene copolymers, isoprene copolymers and the blends of these elastomers. Such copolymers are more preferably chosen from the group consisting of butadiene/styrene copolymers (SBR), isoprene/butadiene copolymers (BIR), isoprene/styrene copolymers (SIR) and isoprene/butadiene/styrene copolymers (SBIR).

According to a specific embodiment, the diene elastomer is predominantly (that is to say, for more than 50 pce) an SBR, whether an SBR prepared in emulsion (“E-SBR”) or an SBR prepared in solution (“S-SBR”), or an SBR/BR, SBR/NR (or SBR/IR) or also BR/NR (or BR/IR) blend. In the case of an SBR elastomer, use is made in particular of an SBR having a styrene content of between 20% and 30% by weight, a content of vinyl bonds of the butadiene part of between 15% and 65%, a content (molar %) of trans-1,4-bonds of between 15% and 75% and a Tg of between −20° C. and −55° C.; such an SBR can advantageously be used as a blend with a BR preferably having more than 90% (molar %) of cis-1,4-bonds.

According to another specific embodiment, the diene elastomer is predominantly (for more than 50 pce) an isoprene elastomer. This is the case in particular when the compositions of the invention are intended to constitute, in the tyres, rubber matrices of certain treads (for example for industrial vehicles), of crown reinforcing plies (for example of working plies, protection plies or hooping plies), of carcass reinforcing plies, of sidewalls, of beads, of protectors, of underlayers, of rubber blocks and other internal rubbers providing the interface between the abovementioned regions of the tyres.

The term “isoprene elastomer” is understood to mean, in a known way, an isoprene homopolymer or copolymer, in other words a diene elastomer chosen from the group consisting of natural rubber (NR), synthetic polyisoprenes (IR), the various copolymers of isoprene and the blends of these elastomers. Mention will in particular be made, among isoprene copolymers, of isobutene/isoprene copolymers (butyl rubber—DR), isoprene/styrene copolymers (SIR), isoprene/butadiene copolymers (BIR) or isoprene/butadiene/styrene copolymers (SBIR). This isoprene elastomer is preferably natural rubber or a synthetic cis-1,4-polyisoprene; use is preferably made, among these synthetic polyisoprenes, of the polyisoprenes having a level (molar %) of cis-1,4-bonds of greater than 90%, more preferably still of greater than 98%.

According to another specific embodiment, in particular when it is intended for a tyre sidewall or for an airtight internal rubber of a tubeless tyre (or other air-impermeable component), the composition in accordance with the invention can comprise at least one essentially saturated diene elastomer, in particular at least one EPDM copolymer or one butyl rubber (optionally chlorinated or brominated), whether these copolymers are used alone or as a blend with highly unsaturated diene elastomers as mentioned above, in particular NR or IR, BR or SBR.

According to another preferred embodiment of the invention, the rubber composition comprises a blend of a (one or more) “high Tg” diene elastomer exhibiting a Tg of between −65° C. and −10° C. and of a (one or more) “low Tg” diene elastomer exhibiting a Tg of between −110° C. and −80° C., more preferably between −105° C. and −90° C. The high Tg elastomer is preferably chosen from the group consisting of S-SBRs, E-SBRs, natural rubber, synthetic polyisoprenes (exhibiting a level (molar %) of cis-1,4-structures preferably of greater than 95%), BIRs, SIRs, SMRs and the blends of these elastomers. The low Tg elastomer preferably comprises butadiene units according to a level (molar %) at least equal to 70%; it preferably consists of a polybutadiene (BR) exhibiting a level (molar %) of cis-1,4-structures of greater than 90%.

According to another specific embodiment of the invention, the rubber composition comprises, for example, from 30 to 100 pce, in particular from 50 to 100 pce, of a high Tg elastomer as a blend with 0 to 70 pce, in particular from 0 to 50 pce, of a low Tg elastomer; according to another example, it comprises, for the whole of the 100 pce, one or more SBR(s) prepared in solution.

According to another specific embodiment of the invention, the diene elastomer of the composition according to the invention comprises a blend of a BR (as low Tg elastomer) exhibiting a level (molar %) of cis-1,4-structures of greater than 90% with an S-SBR or an E-SBR (as high Tg elastomer).

The compositions of the invention can comprise a single diene elastomer or a blend of several diene elastomers, it being possible for the diene elastomer or elastomers to be used in combination with any type of synthetic elastomer other than a diene elastomer, indeed even with polymers other than elastomers, for example thermoplastic polymers.

II-2. Reinforcing Filler

Use may be made of any type of reinforcing filler known for its capabilities of reinforcing a rubber composition which can be used for the manufacture of tyres, for example an organic filler, such as carbon black, a reinforcing inorganic filler, such as silica, or a blend of these two types of filler, in particular a blend of carbon black and silica.

All carbon blacks, in particular blacks of the HAF, ISAF or SAF type, conventionally used in tyres (“tyre-grade” blacks) are suitable as carbon blacks. Mention will more particularly be made, among the latter, of the reinforcing carbon blacks of the 100, 200 or 300 series (ASTM grades), such as, for example, the N115, N134, N234, N326, N330, N339, N347 or N375 blacks, or also, depending on the applications targeted, the blacks of higher series (for example, N660, N683 or N772). The carbon blacks might, for example, be already incorporated in the isoprene elastomer in the form of a masterbatch (see, for example, Applications WO 97/36724 or WO 99/16600).

Mention may be made, as examples of organic fillers other than carbon blacks, of the functionalized polyvinylaromatic organic fillers as described in Applications WO-A-2006/069792 and WO-A-2006/069793.

The term “reinforcing inorganic filler” should be understood, in the present patent application, by definition, as meaning any inorganic or mineral filler, whatever its colour and its origin (natural or synthetic), also known as “white filler”, “clear filler” or even “non-black filler”, in contrast to carbon black, capable of reinforcing by itself alone, without means other than an intermediate coupling agent, a rubber composition intended for the manufacture of tyres, in other words capable of replacing, in its reinforcing role, a conventional tyre-grade carbon black; such a filler is generally characterized, in a known way, by the presence of hydroxyl (—OH) groups at its surface.

The physical state under which the reinforcing inorganic filler is provided is not important, whether it is in the form of a powder, of microbeads, of granules, of beads or any other appropriate densified form. Of course, the term reinforcing inorganic filler is also understood to mean mixtures of different reinforcing inorganic fillers, in particular of highly dispersible siliceous and/or aluminous fillers as described below.

Mineral fillers of the siliceous type, in particular silica (SiO₂), or of the aluminous type, in particular alumina (Al₂O₃), are suitable in particular as reinforcing inorganic fillers. The silica used can be any reinforcing silica known to a person skilled in the art, in particular any precipitated or pyrogenic silica exhibiting a BET surface and a CTAB specific surface both of less than 450 m²/g, preferably from 30 to 400 m²/g. Mention will be made, as highly dispersible (“HD”) precipitated silicas, for example, of the Ultrasil 7000 and Ultrasil 7005 silicas from Degussa, the Zeosil 1165MP, 1135MP and 1115MP silicas from Rhodia, the Hi-Sil EZ150G silica from PPG, the Zeopol 8715, 8745 and 8755 silicas from Huber or the silicas with a high specific surface as described in Application WO 03/16837.

When the compositions of the invention are intended for tyre treads with a low rolling resistance, the reinforcing inorganic filler used, in particular if it is silica, preferably has a BET surface of between 45 and 400 m²/g, more preferably of between 60 and 300 m²/g.

Preferably, the level of total reinforcing filler (carbon black and/or reinforcing inorganic filler, such as silica) is between 20 and 200 pce, more preferably between 30 and 150 pce, the optimum being in a known way different depending on the specific applications targeted: the level of reinforcement expected with regard to a bicycle tyre, for example, is, of course, less than that required with regard to a tyre capable of running at high speed in a sustained manner, for example a motor cycle tyre, a tyre for a passenger vehicle or a tyre for a utility vehicle, such as a heavy duty vehicle.

According to a preferred embodiment of the invention, use is made of a reinforcing filler comprising between 30 and 150 pce, more preferably between 50 and 120 pce, of inorganic filler, particularly silica, and optionally carbon black; the carbon black, when it is present, is preferably used at a level of less than 20 pce, more preferably of less than 10 pce (for example between 0.1 and 10 pce).

In order to couple the reinforcing inorganic filler to the diene elastomer, use is made, in a known way, of an at least bifunctional coupling agent (or bonding agent) intended to provide a satisfactory connection, of chemical and/or physical nature, between the inorganic filler (surface of its particles) and the diene elastomer, in particular bifunctional organosilanes or polyorganosiloxanes.

Use is made in particular of silane polysulphides, referred to as “symmetrical” or “unsymmetrical” depending on their specific structure, as described, for example, in Applications WO03/002648 (or US 2005/016651) and WO03/002649 (or US 2005/016650).

“Symmetrical” silane polysulphides corresponding to the following general formula (III): Z-A-S_(x)-A-Z,  (III) in which:

-   -   x is an integer from 2 to 8 (preferably from 2 to 5);     -   A is a divalent hydrocarbon radical (preferably, C₁-C₁₈ alkylene         groups or C₆-C₁₂ arylene groups, more particularly C₁-C₁₀, in         particular C₁-C₄, alkylenes, especially propylene);     -   Z corresponds to one of the formulae below:

-   -   in which:     -   the R¹ radicals, which are unsubstituted or substituted and         identical to or different from one another, represent a C₁-C₁₈         alkyl, C₅-C₁₈ cycloalkyl or C₆-C₁₈ aryl group (preferably, C₁-C₆         alkyl, cyclohexyl or phenyl groups, in particular C₁-C₄ alkyl         groups, more particularly methyl and/or ethyl),     -   the R² radicals, which are unsubstituted or substituted and         identical to or different from one another, represent a C₁-C₁₈         alkoxyl or C₅-C₁₈ cycloalkoxyl group (preferably a group chosen         from C₁-C₈ alkoxyls and C₅-C₈ cycloalkoxyls, more preferably         still a group chosen from C₁-C₄ alkoxyls, in particular methoxyl         and ethoxyl),         are suitable in particular, without the above definition being         limiting.

In the case of a mixture of alkoxysilane polysulphides corresponding to the above formula (III), in particular the usual mixtures available commercially, the mean value of the “x” index is a fractional number preferably of between 2 and 5, more preferably in the vicinity of 4. However, the invention can also advantageously be carried out, for example, with alkoxysilane disulphides (x=2).

Mention will more particularly be made, as examples of silane polysulphides, of bis((C₁-C₄)alkoxyl(C₁-C₄)alkylsilyl(C₁-C₄)alkyl)polysulphides (in particular disulphides, trisulphides or tetrasulphides), such as, for example, bis(3-trimethoxysilylpropyl) or bis(3-triethoxysilylpropyl)polysulphides. Use is in particular made, among these compounds, of bis(3-triethoxysilylpropyl)tetrasulphide, abbreviated to TESPT, of formula [(C₂H₅O)₃Si(CH₂)₃S₂]₂, or bis(triethoxysilylpropyl)disulphide, abbreviated to TESPD, of formula [(C₂H₅O)₃Si(CH₂)₃S]₂. Mention will also be made, as preferred examples, of bis(mono(C₁-C₄)alkoxyldi(C₁-C₄)alkylsilylpropyl)polysulphides (in particular disulphides, trisulphides or tetrasulphides), more particularly bis(monoethoxydimethylsilylpropyl)tetrasulphide, as described in Patent Application WO 02/083782 (or US 2004/132880).

Mention will in particular be made, as coupling agent other than alkoxysilane polysulphide, of bifunctional POSs (polyorganosiloxanes) or of hydroxysilane polysulphides (R²═OH in the above formula III), such as described in Patent Applications WO 02/30939 (or U.S. Pat. No. 6,774,255) and WO 02/31041 (or US 2004/051210).

In the rubber compositions in accordance with the invention, the content of coupling agent is preferably between 4 and 12 pce, more preferably between 3 and 8 pce.

A person skilled in the art will understand that a reinforcing filler of another nature, in particular organic nature, might be used as filler equivalent to the reinforcing inorganic filler described in the present section, provided that this reinforcing filler is covered with an inorganic layer, such as silica, or else comprises, at its surface, functional sites, in particular hydroxyls, requiring the use of a coupling agent in order to form the connection between the filler and the elastomer.

II-3. Plasticizing System

The rubber compositions of the invention have the essential characteristic of using a plasticizing system comprising, in combination, a plasticizing hydrocarbon resin, the Tg of which is greater than 0° C., and a tri- or pyromellitate ester corresponding to the formula (I), as explained in detail below.

II-3-A. Plasticizing Hydrocarbon Resin

In a way known to a person skilled in the art, the name “plasticizing resin” is reserved in the present patent application, by definition, for a compound which is, on the one hand, solid at ambient temperature (23° C.) (in contrast to the liquid plasticizing compound, such as an oil) and, on the other hand, compatible (that is to say, miscible at the level used, typically of greater than 5 pce) with the rubber composition for which it is intended, so as to act as a true diluting agent.

Hydrocarbon resins are polymers well known to a person skilled in the art which are thus miscible by nature in diene elastomer compositions, when they are additionally described as being “plasticizing”.

They have been widely described in the patents or patent applications mentioned in the introduction to the present document and also, for example, in the work entitled “Hydrocarbon Resins” by R. Mildenberg, M. Zander and G. Collin (New York, VCH, 1997, ISBN 3-527-28617-9), chapter 5 of which is devoted to their applications, in particular in the tyre rubber field (5.5. “Rubber Tires and Mechanical Goods”).

They can be aliphatic or aromatic or also of the aliphatic/aromatic type, that is to say based on aliphatic and/or aromatic monomers. They can be natural or synthetic and may or may not be oil-based (if such is the case, also known under the name of petroleum resins). They are preferably exclusively hydrocarbon, that is to say that they comprise only carbon and hydrogen atoms.

Preferably, the plasticizing hydrocarbon resin exhibits at least one, more preferably all, of the following characteristics:

-   -   a Tg of greater than 20° C.;     -   a number-average molecular weight (Mn) of between 400 and 2000         g/mol;     -   a polydispersity index (PI) of less than 3 (reminder: PI=Mw/Mn         with Mw the weight-average molecular weight).

More preferably, this plasticizing hydrocarbon resin exhibits at least one, more preferably still to all, of the following characteristics:

-   -   a Tg of greater than 30° C.;     -   a weight Mn of between 500 and 1500 g/mol;     -   a PI of less than 2.

The glass transition temperature Tg is measured in a known way by DSC (Differential Scanning Calorimetry) according to Standard ASTM D3418 (1999).

The macrostructure (Mw, Mn and PI) of the hydrocarbon resin is determined by size exclusion chromatography (SEC): solvent tetrahydrofuran; temperature 35° C.; concentration 1 g/l; flow rate 1 ml/min; solution filtered through a filter with a porosity of 0.45 μm before injection; Moore calibration with polystyrene standards; set of 3 “Waters” columns in series (“Styragel” HR4E, HR1 and HR0.5); detection by differential refractometer (“Waters 2410”) and its associated operating software (“Waters Empower”).

According to a particularly preferred embodiment, the plasticizing hydrocarbon resin is chosen from the group consisting of cyclopentadiene (abbreviated to CPD) or dicyclopentadiene (abbreviated to DCPD) homopolymer or copolymer resins, terpene homopolymer or copolymer resins, C₅ fraction homopolymer or copolymer resins and the blends of these resins.

Use is preferably made, among the above copolymer resins, of those chosen from the group consisting of (D)CPD/vinylaromatic copolymer resins, (D)CPD/terpene copolymer resins, (D)CPD/C₅ fraction copolymer resins, terpene/vinylaromatic copolymer resins, C₅ fraction/vinylaromatic copolymer resins and the blends of these resins.

The term “terpene” combines here, in a known way, the α-pinene, β-pinene and limonene monomers; use is preferably made of a limonene monomer, which compound exists, in a known way, in the form of three possible isomers: L-limonene (laevorotatory enantiomer), D-limonene (dextrorotatory enantiomer) or else dipentene, the racemate of the dextrorotatory and laevorotatory enantiomers.

Styrene, α-methylstyrene, ortho-, meta- or para-methylstyrene, vinyltoluene, para-(tert-butyl)styrene, methoxystyrenes, chlorostyrenes, le vinylmesitylene, divinylbenzene, vinylnaphthalene, or any vinylaromatic monomer resulting from a C₉ fraction (or more generally from a C₈ to C₁₀ fraction) are suitable, for example, as vinylaromatic monomer. Preferably, the vinylaromatic compound is styrene or a vinylaromatic monomer resulting from a C₉ fraction (or more generally from a C₈ to C₁₀ fraction). Preferably, the vinylaromatic compound is the minor monomer, expressed as molar fraction, in the copolymer under consideration.

According to a more particularly preferred embodiment, the plasticizing hydrocarbon resin is chosen from the group consisting of (D)CPD homopolymer resins, (D)CPD/styrene copolymer resins, polylimonene resins, limonene/styrene copolymer resins, limonene/(D) CPD copolymer resins, C₅ fraction/styrene copolymer resins, C₅ fraction/C₉ fraction copolymer resins and the blends of these resins.

The preferred resins above are well known to a person skilled in the art and are commercially available, for example sold, as regards the:

-   -   polylimonene resins: by DRT under the name “Dercolyte L120”         (Mn=625 g/mol; Mw=1010 g/mol; PI=1.6; Tg=72° C.) or by Arizona         Chemical Company under the name “Sylvagum TR7125C” (Mn=630         g/mol; Mw=950 g/mol; PI=1.5; Tg=70° C.);     -   C₅ fraction/vinylaromatic, in particular C₅ fraction/styrene or         C₅ fraction/C₉ fraction, copolymer resins: by Neville Chemical         Company under the names “Super Nevtac 78”, “Super Nevtac 85” or         “Super Nevtac 99”, by Goodyear Chemicals under the name         “Wingtack Extra”, by Kolon under the names “Hikorez T1095” and         “Hikorez T1100”, or by Exxon under the names “Escorez 2101” and         “ECR 373”;     -   limonene/styrene copolymer resins: by DRT under the name         “Dercolyte TS 105” or by Arizona Chemical Company under the         names “ZT115LT” and “ZT5100”.

The level of hydrocarbon resin is preferably between 5 and 60 pce. Below the minimum indicated, the targeted technical effect may prove to be inadequate while, above 60 pce, the tackiness of the compositions in the raw state, with regard to the mixing devices, can in some cases become totally unacceptable from the industrial viewpoint. For these reasons, the level of hydrocarbon resin is more preferably between 5 and 40 pce, more preferably still between 10 and 30 pce.

II-3-B.—Tri- or Pyromellitate Ester

The tri- or pyromellitate ester of the plasticizing system of the invention corresponds to the formula (I):

in which:

-   -   the R radicals, which are identical or different, represent any         hydrocarbon radical (or chain) which preferably has from 1 to 30         carbon atoms and which can comprise a heteroatom chosen in         particular from S, O and N;     -   the R′ radical represents hydrogen (case of a trimellitate) or         the COOR group (case of a pyromellitate).

Tri- and pyromellitate esters of formula (I) are known as plasticizers for plastics and various other polymers. They have been described in particular in the article “Oils, Plasticizers and Other Rubber Chemicals” (chapter 8, section 8.2, pages 132-137, of Basic Rubber Testing (2003), Ed. Dick, John S., Publisher—ISBN: 0-8031-3358-8), and have also been described, indeed even simply mentioned among other possible plasticizing agents, in Patent Applications U.S. Pat. No. 4,287,928, EP-A-1 632 364 and WO-A-02/088238, all relating to tyre rubber compositions.

However, to the knowledge of the Applicant Companies, they have never been used in combination with a plasticizing hydrocarbon resin, in particular in such rubber compositions.

Preferably, the R radicals are chosen from the group consisting of linear, branched or cyclic alkyls comprising from 1 to 30 carbon atoms and aryls, aralkyls or alkaryls comprising from 6 to 30 carbon atoms.

More preferably, the R radicals represent a linear, branched or cyclic alkyl group comprising from 1 to 20, more preferably still from 1 to 15, carbon atoms.

Mention may be made, as examples of such preferred R radicals comprising from 1 to 15 carbon atoms, for example of the methyl, ethyl, butoxyethyl, butoxyethoxyethyl, propyl, propenyl, butyl, isobutyl, dibutyl, diisobutyl, benzylbutyl, heptyl, hexyl, ethylhexyl, octyl, isooctyl, benzyloctyl, dioctyl, diisooctyl, nonyl, isononyl, ethylnonyl, isodecyl, diisodecyl, tridecyl or octyldecyl radicals.

Use is preferably made of a trimellitate of formula (II):

in which the R radicals, which are identical or different, represent a hydrocarbon radical having from 1 to 20, more preferably from 1 to 15, carbon atoms.

The corresponding preferred molecular weights are typically included within a range from 252 to 1051 g/mol, more preferably from 252 to 841 g/mol.

Such preferred trimellitates are commercially available; they have been developed essentially for the plasticizing of rigid plastics, such as PVC. Mention may be made, by way of examples, of the trimellitates of the “Jayflex” series sold by Exxon Mobil, in particular the trimellitates “Jayflex TIOTM” (R═C₈H₁₇ in the formula II) or “Jayflex TINTM” (R═C₉H₁₉ in the formula II).

Mention may be made, as other examples of these trimellitates of formula (II), of the plasticizers of the “Palatinol” series from BASF, for example “TOTM” (R═C₈H₁₇), “TNTM” (R═C₉H₁₉) or “79TM-I” (triethylnonyl), “Plasthall” from CP HALL or “Diplast” from Lonza.

Use is very particularly made of trioctyl trimellitate (abbreviated to “TOTM” or tri(2-ethylhexyl)trimellitate) or triisooctyl trimellitate (abbreviated to “TIOTM”), both of formula (II) in which R represents the C₈H₁₇ group, or a mixture of these two compounds TOTM and TIOTM.

In the rubber composition of the invention, the level of tri- or pyromellitate is preferably between 5 and 60 pce. Below the minimum indicated, the targeted technical effect may prove to be inadequate while, above 60 pce, there is a risk of a reduction in grip of the tyres when the compositions of the invention are used in the treads of these tyres. For these reasons, the level of tri- or pyromellitates is more preferably between 5 and 40 pce, more preferably still between 10 and 30 pce.

With regard to the overall level of plasticizing system according to the invention in the rubber composition of the invention, it is preferably between 10 and 100 pce, more preferably between 20 and 80 pce (in particular between 20 and 50 pce).

All the tri- or pyromellitates described in the present section are liquid at ambient temperature (23° C.). They exhibit a Tg typically of less than −80° C. Therefore, according to a specific embodiment of the invention, they could be used, in all or part, as extending oil for the diene elastomers present in the rubber composition of the invention.

II-4. Various Additives

The rubber compositions in accordance with the invention can also comprise all or a portion of the usual additives generally used in elastomer compositions intended for the manufacture of tyres or semi-finished products for tyres, such as, for example, other plasticizing agents (other than the plasticizing system of the invention), preferably non-aromatic or very slightly aromatic plasticizing agents, for example naphthenic or paraffinic oils, IVIES or TDAE oils, glycerol esters (in particular trioleates), especially natural esters, such as rapeseed or sunflower vegetable oils, pigments, protection agents, such as antiozone waxes, chemical antiozonants, antioxidants, antifatigue agents, reinforcing resins, methylene acceptors (for example, phenolic novolak resin) or methylene donors (for example, HMT or H3M), a crosslinking system based either on sulphur or on sulphur donors and/or on peroxide and/or on bismaleimides, vulcanization accelerators, vulcanization activators or antireversion agents.

These compositions can also comprise, in addition to coupling agents, coupling activators, agents for covering the inorganic fillers or more generally processing aids capable, in a known way, by virtue of an improvement in the dispersion of the filler in the rubber matrix and of a lowering in the viscosity of the compositions, of improving their ability to be processed in the raw state, these agents being, for example, hydrolysable silanes, such as alkylalkoxysilanes, polyols, polyethers, primary, secondary or tertiary amines or hydroxylated or hydrolysable polyorganosiloxanes.

II-5. Manufacture of the Rubber Compositions

The compositions are manufactured in appropriate mixers using two successive preparation phases well known to a person skilled in the art: a first phase of thermomechanical working or kneading (“non-productive” phase) at high temperature, up to a maximum temperature of between 110° C. and 190° C., preferably between 130° C. and 180° C., followed by a second phase of mechanical working (“productive” phase) up to a lower temperature, typically of less than 110° C., for example between 40° C. and 100° C., finishing phase during which the crosslinking system is incorporated.

The process in accordance with the invention for preparing a rubber composition exhibiting in particular an improved wear resistance comprises the following stages:

-   -   incorporating in a diene elastomer, during a first stage         (“non-productive” stage), at least one reinforcing filler and         one plasticizing system, everything being kneaded         thermomechanically, in one or more goes, until a maximum         temperature of between 110° C. and 190° C. is reached;     -   cooling the combined mixture to a temperature of less than 100°         C.;     -   subsequently incorporating, during a second stage (“productive”         stage), a crosslinking system;     -   kneading everything up to a maximum temperature of less than         110° C.,         and it is characterized in that the said plasticizing system         comprises, in combination, a plasticizing hydrocarbon resin, the         Tg of which is greater than 0° C., and a tri- or pyromellitate         corresponding to the abovementioned formula (I), preferably a         trimellitate corresponding to the abovementioned formula (II) in         which the R radicals, which are identical or different,         represent a hydrocarbon radical having from 1 to 20 carbon         atoms.

By way of example, the non-productive phase is carried out in a single thermomechanical stage during which, in a first step, all the necessary base constituents (diene elastomer, reinforcing filler and coupling agent, if necessary, plasticizing system) are introduced into an appropriate mixer, such as a normal internal mixer, followed, in a second step, for example after kneading for one to two minutes, by the other additives, optional additional covering agents or processing aids, with the exception of the crosslinking system. After cooling the mixture thus obtained, the crosslinking system is then incorporated in an external mixer, such as an open mill, maintained at a low temperature (for example, between 40° C. and 100° C.). The combined mixture is then mixed (productive phase) for a few minutes, for example between 2 and 15 min.

The crosslinking system is preferably a vulcanization system based on sulphur and on an accelerator. Use may be made of any compound capable of acting as accelerator of the vulcanization of diene elastomers in the presence of sulphur, in particular those chosen from the group consisting of 2-mercaptobenzothiazyl disulphide (abbreviated to “METS”), N-cyclohexyl-2-benzothiazolesulphenamide (abbreviated to “CBS”), N,N-dicyclohexyl-2-benzothiazolesulphenamide (abbreviated to “DCBS”), N-tert-butyl-2-benzothiazolesulphenamide (abbreviated to “TBBS”), N-tert-butyl-2-benzothiazolesulphenimide (abbreviated to “TBSI”) and the mixtures of these compounds. Preferably, a primary accelerator of the sulphenamide type is used.

Additional to this vulcanization system are various known secondary accelerators or vulcanization activators, such as zinc oxide, stearic acid, guanidine derivatives (in particular diphenylguanidine), and the like, incorporated during the first non-productive phase and/or during the productive phase. In the case of use of the composition of the invention as tyre tread, the level of sulphur is, for example, between 0.5 and 3.0 pce and that of the primary accelerator is between 0.5 and 5.0 pce.

The final composition thus obtained is subsequently calendered, for example in the form of a sheet or of a plaque, in particular for characterization in the laboratory, or else extruded in the form of a rubber profiled element which can be used, for example, as a tyre tread for a passenger vehicle.

The vulcanization (or curing) is carried out in a known way at a temperature generally of between 130° C. and 200° C. for a sufficient time which can vary, for example, between 5 and 90 min depending in particular on the curing temperature, the vulcanization system adopted and the vulcanization kinetics of the composition under consideration.

The invention relaters to the rubber compositions described above both in the “raw” state (i.e., before curing) and in the “cured” or vulcanized state (i.e. after vulcanization).

III. EXAMPLES OF THE IMPLEMENTATION OF THE INVENTION

III-1. Preparation of the Rubber Compositions and Treads

The tests which follow are carried out in the following way: the reinforcing filler, the coupling agent, the plasticizing system, the diene elastomer and the various other ingredients, with the exception of the vulcanization system, are successively introduced into an internal mixer, 70% filled and having an initial vessel temperature of approximately 60° C. Thermomechanical working (non-productive phase) is then carried out in one stage, which lasts in total approximately from 3 to 4 minutes, until a maximum “dropping” temperature of 165° C. is reached.

The mixture thus obtained is recovered and cooled and then sulphur and an accelerator of sulphenamide type are incorporated on an external mixer (homofinisher) at 30° C., the combined mixture being mixed (productive phase) for an appropriate time (for example, between 5 and 12 min).

The compositions thus obtained are subsequently calendered, either in the form of plaques (thickness of 2 to 3 mm) or of fine sheets of rubber, for the measurement of their physical or mechanical properties, or extruded in the form of treads for passenger vehicle tyres.

III-2. Rubber Tests and Tyre Running Tests

The aim of these tests is to demonstrate the improved performance of a rubber composition according to the invention, in comparison with a control composition of the prior art.

For this, two compositions, denoted C-1 and C-2, based on diene elastomers (SSBR and BR blends) reinforced with silica and carbon black are prepared. The two compositions are prepared in a mixer which is sufficiently big to make possible the manufacture of treads and the performance of running tests on tyres comprising these treads.

The two compositions tested are identical, except for the plasticizing system used, which comprises, in combination, one and the same plasticizing hydrocarbon resin (polylimonene) as first plasticizing agent and two other types of compound as second plasticizing agent:

-   -   composition C-1: polylimonene resin+MES oil;     -   composition C-2: polylimonene resin+trimellitate.

As explained in the introduction of the present document, composition C-1 is a reference composition for the Applicant Companies which has furtheimore given proof of its excellent performance in terms of wear or abrasion resistance. The MES (for “Medium Extracted Solvates”) oil is an oil of the “non-aromatic” type which is characterized by a very low level of polyaromatics (approximately 20 to 50 times less), in comparison with conventional petroleum-derived aromatic oils which comprise a high level of aromatics, known under the name of DAE (for “Distillate Aromatic Extracts”) oils.

Composition C-2, comprising the plasticizing system in accordance with the invention, is thus itself in accordance with the invention.

Tables 1 and 2 give the make-up of the two compositions (Table 1—level of the various products expressed in “pce” or parts by weight per one hundred parts of elastomer(s)) and their properties before and after curing (30 min at 150° C.); the vulcanization system is composed of sulphur and sulphenamide.

The examination of the various results in Table 2 shows that the rubber properties measured are substantially equivalent from one composition to the other, before and after curing.

Nothing in these rubber properties thus allowed a person skilled in the art to anticipate the improved performance which was observed for the composition of the invention, as is explained in detail below.

The two compositions were tested as treads of radial carcass passenger vehicle tyres, with a size of 195/65 R15 (speed rating H), conventionally manufactured and in all respects identical apart from the constituent rubber composition of the tread: composition C-1 for the control tyres (denoted T-1) and composition C-2 for the tyres of the invention (denoted T-2).

The tyres were tested in accordance with the instructions in the preceding section 1-6 (“Renault Laguna” vehicle for the grip test; “Mercedes C200” vehicle for the wear resistance test).

The rolling results obtained are given in the appended Table 3.

They clearly demonstrate that the T-2 tyres of the invention offer the best compromise in properties with very particularly a significantly increased (+7%) wear resistance with respect to the T-1 tyres and to the plasticizing system (polylimonene+MES oil), which nevertheless constitutes a reference with regard to the criterion of wear resistance (see abovementioned Application WO-A-2005/087859). It should be emphasized that this improvement is obtained without having a detrimental affect on the other properties, in particular that of wet grip.

Other control tyres (T-3) were prepared using, in combination, a polylimonene resin and another plasticizing agent of the family of the ester plasticizing agents, in this case an alkyl oleate (“Plasthall 7049” from CP HALL) as described, for example, in the abovementioned Application WO-A02/088238.

The wear resistance test demonstrated a wear resistance for the T-2 tyres in accordance with the invention greater by 17% in comparison with the T-3 tyres, which altogether confirms the superiority of the trimellitate plasticizing agent over another plasticizing agent of ester type in the presence of one and the same plasticizing hydrocarbon resin.

In conclusion, the novel plasticizing system of the invention offers tyre rubber compositions a particular advantageous compromise in properties, with an improved wear resistance without having a detrimental affect on the other properties, in particular that of wet grip.

TABLE 1 Composition No: C-1 C-2 SBR (1) 70 70 BR (2) 30 30 Silica (3) 80 80 Coupling agent (4) 6.4 6.4 Carbon black (5) 6 6 Plasticizing agent (6) 15 15 Plasticizing agent (7) 14 — Plasticizing agent (8) — 14 DPG (9) 1.5 1.5 ZnO 2.5 2.5 Stearic acid 2 2 Antiozone wax 1.5 1.5 Antioxidant (10) 1.9 1.9 Sulphur 1.1 1.1 Accelerator (11) 2.0 2.0 (1) SSBR with 25% of styrene, 64% of 1,2-polybutadiene units and 25% of trans-1,4-polybutadiene units (Tg = −18° C.); (2) BR with 4.3% of 1,2-; 2.7% of trans-1,4-; 93% of cis-1,4- (Tg = −106° C.); (3) Silica “Zeosil 1165MP” from Rhodia, “HDS” type (BET and CTAB: approximately 160 m²/g); (4) Coupling agent TESPT (“Si69” from Degussa); (5) Carbon black N234 (ASTM grade); (6) Polylimonene resin (“Dercolyte L120” from DRT); (7) MES oil (Catenex SNR from Shell); (8) Trioctyl trimellitate (“Diplast TM/ST” from Lonza); (9) Diphenylguanidine (Perkacit DPG from Flexsys); (10) N-(1,3-Dimethylbutyl)-N-phenyl-para-phenylenediamine (Santoflex 6-PPD from Flexsys); (11) CBS (Santocure from Flexsys).

TABLE 2 Composition No.: C-1 C-2 Properties before curing: Mooney (UM) 80 74 t_(i) (min) 6.6 7.0 t₉₀ (min) 26.8 27.2 t₉₀-t_(i) (min) 20.2 20.2 K (min⁻¹) 0.113 0.114 Properties after curing: Shore A hardness 68.6 68.4  M10 (MPa) 6.1 5.9 M100 (MPa) 2.0 2.1 M300 (MPa) 2.5 2.6 M300/M100 1.25 1.24 tan(δ)max (40° C.) 0.291 0.280 Breaking stress (MPa) 20.3 20.0 Elongation at break (%) 477 486

TABLE 3 Properties (in relative units) T-1 T-2 Rolling resistance 100 102 Wear resistance 100 107 Wet grip 100 100 (a value of greater than 100 indicates an improved performance with respect to the control T-1 of base 100) 

The invention claimed is:
 1. A rubber composition comprising at least one diene elastomer, one reinforcing filler and one plasticizing system, wherein the plasticizing system comprises: a plasticizing hydrocarbon resin, the glass transition temperature (Tg) of which is greater than 0° C.; and a tri- or pyromellitate ester corresponding to the formula (I):

wherein the R radicals, which are identical or different, represent a hydrocarbon radical and the R′ radical represents hydrogen or COOR group.
 2. The rubber composition according to claim 1, wherein the R radicals, which are identical or different, are selected from the group consisting of linear alkyls comprising from 1 to 30 carbon atoms, branched alkyls comprising from 1 to 30 carbon atoms, cyclic alkyls comprising from 1 to 30 carbon atoms, aryls comprising from 6 to 30 carbon atoms, aralkyls comprising from 6 to 30 carbon atoms, and alkaryls comprising from 6 to 30 carbon atoms.
 3. The rubber composition according to claim 2, wherein the R radicals represent a linear, branched or cyclic alkyl group comprising from 1 to 20 carbon atoms.
 4. The rubber composition according to claim 3, wherein the ester is a trimellitate of formula (II):


5. The rubber composition according to claim 4, wherein the trimellitate is selected from the group consisting of trioctyl trimellitate (TOTM), triisooctyl trimellitate (TIOTM), and mixtures thereof.
 6. The rubber composition according to claim 1, wherein the tri or pyromellitate ester is present at a level of between 5 and 60 pce.
 7. The rubber composition according to claim 1, wherein the Tg of the plasticizing hydrocarbon resin is greater than +20° C.
 8. The rubber composition according to claim 1, wherein the plasticizing hydrocarbon resin has a number-average molecular weight of between 400 and 2000 g/mol.
 9. The rubber composition according to claim 1, wherein the plasticizing hydrocarbon resin has a polydispersity index of less than
 3. 10. The rubber composition according to claim 1, wherein the plasticizing hydrocarbon resin is selected from the group consisting of cyclopentadiene (abbreviated to CPD) homopolymer resin, CPD copolymer resin, dicyclopentadiene (abbreviated to DCPD) homopolymer resin, DCPD copolymer resin, terpene homopolymer resin, terpene copolymer resin, C₅ fraction homopolymer resin, C₅ fraction copolymer resin, and blends thereof.
 11. The rubber composition according to claim 10, wherein the plasticizing hydrocarbon resin is selected from the group consisting of CPD homopolymer resin, DCPD homopolymer resin, CPD/styrene copolymer resin, DCPD/styrene copolymer resin, polylimonene resin, limonene/styrene copolymer resin, limonene/CPD copolymer resin, limonene/DCPD copolymer resin, C₅ fraction/styrene copolymer resin, C₅ fraction/C₉ fraction copolymer resin, and blends thereof.
 12. The rubber composition according to claim 1, wherein the plasticizing hydrocarbon resin is present in a level of between 5 and 60 pce.
 13. The rubber composition according to claim 1, wherein the at least one diene elastomer is selected from the group consisting of polybutadienes, synthetic polyisoprenes, natural rubber, butadiene copolymers, isoprene copolymers, and blends thereof.
 14. The rubber composition according to claim 1, wherein the tri- or pyromellitate ester is trioctyl trimellitate.
 15. The rubber composition according to claim 14, wherein the tri- or pyromellitate ester is present in an amount between 5 and 40 pce.
 16. The rubber composition according to claim 15, wherein the amount of the tri- or pyromellitate ester is between 10 and 30 pce.
 17. The rubber composition according to claim 1, wherein the tri- or pyromellitate ester is present in an amount between 5 and 40 pce.
 18. The rubber composition according to claim 17, wherein the amount of the tri- or pyromellitate ester is between 10 and 30 pce.
 19. The rubber composition according to claim 5, wherein the tri- or pyromellitate ester is present in an amount between 5 and 40 pce.
 20. The rubber composition according to claim 19, wherein the amount of the tri- or pyromellitate ester is between 10 and 30 pce.
 21. A tire tread comprising the rubber composition according to claim
 1. 22. A tire comprising the tread according to claim
 21. 23. A process for preparing a rubber composition comprising at least one diene elastomer, one reinforcing filler and one plasticizing system, the process comprising the following stages: incorporating in the at least one diene elastomer, in a mixer: the reinforcing filler; the plasticizing system; kneading everything thermomechanically, in one or more goes, until a maximum temperature of between 110° C. and 190° C. is reached; cooling the combined mixture to a temperature of less than 100° C.; subsequently incorporating: a crosslinking system; kneading everything up to a maximum temperature of less than 110° C.; and extruding or calendering the rubber composition thus obtained; and wherein the plasticizing system comprises at least: one plasticizing hydrocarbon resin, the glass transition temperature (Tg) of which is greater than 0° C.; and a tri- or pyromellitate ester corresponding to the formula (I):

wherein the R radicals, which are identical or different, represent a hydrocarbon radical and the R′ radical represents hydrogen or COOR group. 